Thecomplianceofthelungsisaffectedbyseveralrespiratorydisorders.Inemphysema,anobstructivelungdiseasethatusuallyoccursinpeoplewhosmokeandisassociatedwithdestructionofthealveolarseptaandpulmonarycapillarybed,thelungismorecompliant;thatis,forevery1–cmH2Oincreaseinpressure,theincreaseinvolumeisgreaterthanthatinanormallung(see
Fig.21.1 ).Incontrast,inpulmonaryfibrosis,arestrictivelungdiseaseassociatedwithincreasedcollagenfiberdepositionintheinterstitialspace,thelungisnoncompliant;thatis,forevery1–cmH2Ochangeinpressure,thechangeinvolumeisless.Thesechangesincomplianceareofclinicalsignificancebecausealungwithlowcompliancerequireslargerchangesinpleuralpressuretoeffectchangesinlungvolumes;asresult,theworkofbreathingisincreasedforeverybreaththattheindividualtakes.
1 liter = 0.2 0.5 liter = 0.1 0.1 liter = 0.02 = 0.2 5 cm H2O 0.1 liter 0.2 = 0.2 0.1 = 0.2 0.5 liter 0.02 •Fig. 21.9 Relationship Between Compliance and Lung Volume. Imaginelungsinwhicha5–cmH2Ochangeinpressureresultsina1-Lchangeinvolume(situation1).Ifonelungisremoved(situation2),thecompliancedecreases,butwhencorrectedforvolumeofthelung,thereisnochange(specificcompliance).Evenwhentheremaininglungisreducedby90%(situation3),thespecificcomplianceisunchanged.
effect on surface tension. Surface tension is a force caused by water molecules at the airliquid interface that tends to minimize surface area, which makes inflating the lungs more difficult. The effect of surface tension on lung inflation is illustrated by a comparison of the volumepressure curves of a salinefilled lung and of an airfilled lung. Higher pressure is necessary to fully inflate the lung with air than with saline because of the higher surface tension forces in airfilled lungs than in salinefilled lungs. Surface tension is a measure of the attractive force of the surface molecules per unit length of material to which they are attached. The units of surface tension are those of a force applied per unit length. For a sphere (such as an alveolus), the relationship between the pressure within the sphere (Ps) and the tension in the wall is described by the law of Laplace:
Equation 21.7 where T is the wall tension (in dynes per centimeter) and r is the radius of the sphere.
The alveoli are lined with a predominantly lipidbased substance called surfactant. Pulmonary surfactant serves several physiological roles, including (1) reducing the work of breathing by decreasing surface tension forces; (2) preventing collapse and sticking of alveoli on exhalation; and (3) stabilizing alveoli, especially those that tend to deflate at low surface tension. In the absence of surfactant, the surface tension at the airliquid interface would remain constant, and the transalveolar pressure needed to keep it at that volume would be higher when alveolar volumes are lower (
Fig. 21.10A ). Therefore, higher transalveolar pressure would be necessary to produce a given increase in alveolar volume at lower lung volumes than at higher lung volumes. Surfactant stabilizes the inflation of alveoli because it allows the surface tension to increase as the alveoli become larger (see
Fig. 21.10B ). As a result, the transalveolar pressure necessary to keep an alveolus inflated increases as lung volume and transpulmonary pressure increases, and it decreases as lung volume decreases. In the presence of surfactant, surface tension is increased at high lung volume and decreased at low lung volume. The result is that the lungs can maintain alveoli at many different volumes. Otherwise, the gas in small alveoli would empty into large alveoli.